An electromechanically operable parking brake is, for example, known from German Offenlegungsschrift DE 41 29 919 A1. This publication discloses a combination of an electric motor that acts as an actuator and a drum brake. The electric motor is arranged in the vicinity of the associated wheel brake or forms a structural unit together with the wheel brake.
In German application serial No. 197 12 046.7, which is not prior published, the above-mentioned parking brake has been improved in such a way that the rotor of the electric motor has a hollow design and radially encompasses a reduction gear coupled between the rotor and the brake unit. In this arrangement, the reduction gear is configured as a spindle drive, the spindle of which is coupled to the brake unit in a force-transmitting fashion, and the spindle nut of which is coupled to the rotor in a force-transmitting fashion. The spindle drive may be realized in a self-locking fashion or in a non-self-locking fashion. In the latter solution, an additional mechanical or electromechanical locking mechanism may be provided.
Further, a control unit for actuating, for example, an electric motor of a parking brake, which motor is designed as a d.c. motor, is disclosed in German application serial No. 198 17 891.3 which is not prior published. The control unit produces a pulse-width modulated corrective signal for the d.c. motor which provides a nominal pulse-duty factor independently of the magnitude of the supply voltage that actually prevails.
Basically, the electric motor is supplied with an alternating current of a defined amplitude in the case of an electric a.c. motor or, for example, with a pulse-width modulated corrective signal in the case of an electric d.c. motor until the parking brake has reached a defined locking force. The attainable locking force depends on the current rate supplied. For example, a locking force of 1800 to 2000 N can be realized for a current rate of 20 ampere in the parking brakes.
An object of the present invention is to increase the locking force while the current supplied is minimized.
In the method of the present invention, the current supplied to the electric motor is basically interrupted at least one time during the locking and/or release operation. A control unit in the parking brake of the present invention is designed so that it basically interrupts the current supplied to the actuator at least one time during the parking and/or release operation.
More specifically, this means that for one individual locking or release operation, current is delivered to the actuator for a defined period of time, and subsequently is interrupted for a defined duration or at least decreased so greatly that this can be called an interruption, and finally the current is supplied again for a defined time. These interruptions may be repeated, if necessary.
In this arrangement, it is insignificant in which way the current is supplied to the actuator. Alternating current is supplied when the actuator is configured as an a.c. motor, and a pulse-width modulated current or a direct current is supplied when the actuator is a d.c. motor.
Principally, a device is called an actuator which executes a mechanical movement when a current is applied. The device may be an electric motor, an electric magnet with a movable armature plate, or a similar element. The following description refers only to the case of an electric motor being the actuator, which should not be considered a limitation though.
The pauses of a pulse-width modulated corrective signal are not generally considered as an interruption in the sense of the present invention. The pulses sent in pulse-width modulation are repeated in a frequency which is so high that the d.c. motor which acts as a low-pass filter is constantly driven, and thus cannot follow the pulse variations by way of a change in motion. This is the reason why it is not generally possible in this case to speak of an interruption of the current supplied.
Advantageously, a great amount of locking force at a minimum rate of current is achieved in the present invention. The result is that especially the costs in terms of power supply can be minimized.
It is preferred that initially the actuator is furnished uninterruptedly with a current up to a defined holding value of the locking force. Typically, the current which flows through an electric motor in the capacity of an actuator when starting from standstill proceeds in such a manner that after an initial maximum current amplitude for start-up of the motor there follows a current amplitude drop at low load which rises with increasing locking force due to the rising load. The locking force is increased up to a determined holding value. Due to the uninterrupted current supply up to this holding value, advantageously, the brake is quickly actuated and not stopped in positions in which the parking brake has not yet been tightened.
Preferrably, the current supplied to the actuator is repeatedly interrupted when the holding value is reached. It is advantageously achieved thereby that the locking force can be increased successively to very high values.
Preferrably, the pauses of interruption are chosen to be so long that the locking force exerted by the brake unit adopts a constant value in the pauses. With this variation of the locking force, the material which is stressed by the locking operation is given sufficient time for relief so that the electric motor, due to the clearances of parts of the transmission mechanism which occur caused by the material relief, is able to briefly accelerate and, thus, can supply a sufficient torque for the following locking pulse.
Preferrably, a stepped variation of the locking force is adjusted by the appropriate selection of the interruption period and/or the duration of the renewed current supply. Advantageously, this measure permits reaching an optimum in the total time for the locking operation and the attainable locking force depending on the accelerated masses and the material elasticity.
Preferrably, the rate of current supplied to the actuator is limited towards the top. In a combination with the locking operation according to the present invention, the same amount of locking force can be achieved with a rate of current that is reduced compared to the state of the art. The maximum current flowing can be limited in such a fashion that the electric motor can advantageously be driven with a low-cost control unit. This control unit requires only inexpensive power transistors, for example, instead of the costly power transistors used in a pulse-width modulation. Especially with respect to the back-up battery which is necessary for an emergency actuation this permits saving costs drastically. When lithium cells are e.g. used as a back-up battery in the prior art, the present invention permits dividing the number of battery cells by two, compared to the state of the art. More particularly, the need for a pulse width modulation is eliminated.